1. Field of Invention
The present invention relates to semiconductor light emitting devices, and more particularly to improve the light output of the active region of a semiconductor light emitting device.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. III-nitride devices formed on conductive substrates may have the p- and n-contacts formed on opposite sides of the device. Often, III-nitride devices are fabricated on insulating substrates, such as sapphire, with both contacts on the same side of the device. Such devices are mounted so light is extracted either through the contacts (known as an epitaxy-up device) or through a surface of the device opposite the contacts (known as a flip chip device).
The crystal layers in III-nitride devices are often grown as strained wurtzite crystals on lattice-mismatched substrates such as sapphire. Such crystals exhibit two types of polarization: spontaneous polarization, which arises from the crystal symmetry, and piezoelectric polarization, which arises from strain. The total polarization in a layer is the sum of the spontaneous and piezoelectric polarization.
FIG. 1A is a sectional view schematically illustrating a typical conventional strained wurtzite nitride double heterostructure semiconductor, described in U.S. Pat. No. 6,515,313. According to U.S. Pat. No. 6,515,313, the illustrated substrate layer 1 may be any material suitable for growing nitride semiconductors, including spinel (MgAl2O4), sapphire (Al2O3), SiC (including 6H, 4H, and 3C), ZnS, ZnO, GaAs, AlN and GaN. The substrate thickness typically ranges from 100 μm to 1 mm. A buffer layer 2 on the substrate 1 can be formed of AlN, GaN, AlGaN, InGaN or the like. The buffer layer accomodates possible lattice mismatches between the substrate 1 and an overlying conductive contact layer 3. However, the buffer layer 2 may be omitted if the substrate has a lattice constant approximately equal to that of the nitride semiconductor. The buffer layer 2 may also be omitted with some nitride growth techniques. Depending upon the material composition, the buffer layer energy bandgap may range from 2.1 eV to 6.2 eV, with a thickness of about 0.5 μm to 10 μm. See U.S. Pat. No. 6,515,313, column 2, lines 31–48.
The n-type contact layer 3 is also typically formed from a nitride semiconductor, preferably GaN or InGaN with a thickness ranging from 0.5 μm to 5.0 μm, and a bandgap of approximately 3.4 eV for GaN and less for InGaN (depending upon the Indium concentration). A lower n-type or undoped cladding layer 4 on the conductive layer 3 conventionally comprises GaN or AlGaN, with a bandgap of 3.4 eV for GaN and greater for AlGaN (depending upon the Al concentration). Its thickness can range from 1 nm to 100 nm. See U.S. Pat. No. 6,515,313, column 2, lines 49–58.
Nitride double heterostructures typically employ InGaN as an active region 5 over the lower cladding layer, with a thickness of 1 nm to 100 nm. The bandgap of this layer is typically 2.8 eV for blue emission, but may vary depending upon the Indium concentration. A top p-type or undoped cladding layer 6 over the active region is generally comprised of AlGaN or GaN, with a thickness and bandgap energy similar to that of the lower n-type cladding layer 4. A p-type GaN conductive contact layer 7 on the cladding layer 6 has an energy bandgap of about 3.4 eV and a thickness of about 10 nm to 500 nm. A polarization-induced sheet charge occurs at the interface between layers due to different constituent materials. Of particular concern for the operation of a light emitter are the polarization-induced sheet charges adjacent to the active region 5. See U.S. Pat. No. 6,515,313, column 2 line 59 to column 3 line 7.
With the compound semiconductor illustrated in FIG. 1A, a negative polarization-induced sheet charge density σ1, with a magnitude such as 1013 electrons/cm2, is typically formed at the interface between the active region 5 and the lower cladding layer 4. A positive sheet charge density σ2 of similar magnitude is formed at the interface between the active region 5 and the upper cladding layer 6. The polarities and magnitudes of these charges depend upon the crystal symmetry and composition differences of the crystal layers. In general, the density of a sheet charge will depend upon both the spontaneous polarization and the piezoelectric polarization due to strain between the two adjacent layers. For example, σ1 between an In0.2Ga0.8N active region 5 and a GaN cladding layer 4 is about 8.3×1012 electrons/cm2. See U.S. Pat. No. 6,515,313, column 3, lines 8–26.
FIG. 1B illustrates the energy bands corresponding to the device structure of FIG. 1A. When the device is operating, the naturally occurring polarization field across the quantum well generated by σ1 and σ2 reduces the efficiency in a number of ways. First, the electric field in the quantum well leads to a spatial separation (movement in the opposite direction) of electrons and holes within the region. As illustrated, holes in the valence band Ev are attracted to the negative sheet charge σ1 at one end of the active region 5, while electrons in the conduction band Ec are attracted to the positive sheet charge σ2 at its other end. This spatial separation of carriers lowers the probability of radiative recombination, reducing light emission efficiency. Second, the energy barriers of the conduction and valence band are reduced by tilting of the band associated with the electric field. Thus, carriers below Ev and above Ec escape the well through the paths indicated by dashed lines A. Third, the presence of polarization-induced fields also leads to carrier overshoots, illustrated by carrier trajectories B, from the higher Ec level on the σ1 side of the active region to the lower Ec level on the σ2 side, and from the lower Ev level on the σ2 side of the active region to the higher Ev level on the σ1 side. See U.S. Pat. No. 6,515,313, column 3, line 56 to column 4 line 10.
Another issue of concern for applications engineers is the stability of the emission wavelength as the applied bias is increased. If strong polarization-induced fields are present, the emission wavelength will blue-shift as the device bias is increased. As the device bias is increased, more carriers accumulate in the conduction and valence band wells. Since the carriers are spatially separated, they will themselves form a dipole that opposes, or screens, the built-in polarization induced field. As the net electric field is reduced, the quantized energy states of the quantum wells change, resulting in a blue-shift of the emission wavelength. See U.S. Pat. No. 6,515,313, column 4, lines 11–21.
In order to reduce or cancel the effect of the crystal's naturally occurring polarization induced charges to improve carrier confinement, to reduce their spatial separation, and to reduce carrier overshoot, U.S. Pat. No. 6,515,313 proposes that one or more layers in or around the active region be graded in composition or doping to generate space charges that oppose the polarization-induced charges. Specifically, at column 10, lines 31–34, U.S. Pat. No. 6,515,313 teaches “the active region has a continuously graded Indium concentration from a low of 5% to a high of 10%, with a gradient of approximately 1%/nm” or 0.1%/Å.
Published U.S. Application 2003/0020085, application Ser. No. 09/912,589, filed Jul. 24, 2001, assigned to the assignee of the present application, and incorporated herein by reference, also proposes composition grading in the active region. Paragraph [0036] teaches “quantum well layer 40 is about 40 Å thick, and the mole fraction of indium in InxGa1−xN quantum well layer 40 grades linearly from a mole fraction of about x=0.4 near its interface with barrier layer 38 to a mole fraction of about x=0 near its interface with barrier layer 42[,]” corresponding to a grading slope or gradient of 1%/Å.